Along with the development of storage technology, various memories are invented. Digital data can be written/programmed into the memory and then those digital data can be read out.
The reading method of the memory may cause nearby cells in the same memory block to change over time. This is known as read disturb (or read disturbance). To avoid the read disturbance problem, the controller may typically count the total number of reads to a block since the last erase. When the total number of reads exceeds a target limit, the data stored in the affected block will be copied to a new block, and then the affected block is erased. The affected block is as good as new after the erasing. However, it costs time to copy the data of the affect block and some of the data may be lost before the total number of reads exceeds the target limit.